GB2430040A

GB2430040A - Determining the earth loop resistance of a mains supply

Info

Publication number: GB2430040A
Application number: GB0614434A
Authority: GB
Inventors: David Robert Harris
Original assignee: KEW TECHNIK Ltd
Current assignee: KEW TECHNIK Ltd
Priority date: 2005-07-20
Filing date: 2006-07-20
Publication date: 2007-03-14
Anticipated expiration: 2026-07-20
Also published as: GB0614434D0; GB0514842D0; GB2430040B

Abstract

A tester for and method of determining the line-earth loop resistance or impedance of a mains supply, which includes a line resistance, a neutral resistance and an earth resistance, and preferably at least one other characteristic of the mains supply. In one embodiment the tester comprises: measurement means for measuring line-neutral voltages both with and without a high-current load being applied across the line and neutral resistances of the mains supply, and neutral-earth voltages both with and without a low current being injected into the neutral and earth resistances of the mains supply; and determination means for determining the line-neutral resistance from the measured line-neutral voltages, a difference resistance, which represents a difference between the earth resistance and the neutral resistance, from the measured neutral-earth voltages, and the line-earth resistance from the sum of the line-neutral resistance and the difference resistance.

Description

TESTING DEVICE AND METHOD

The present invention relates to a tester and method for testing at least the line-earth loop resistance or Impedance of a mains supply, In particular a TN mains supply, which can incorporate a residual current detector (RCD), such as an earth-leakage circuit breaker.

Numerous line-earth loop resistance testers are available for measuring the line-earth loop resistance of mains supplies which incorporate residual current detectors. Such testers are disclosed, for example, In EP-A- 0881500, EP-A-0909956 and EP-A-1306682.

It is an aim of the present Invention to provide an improved tester and method for testing at least the line-earth loop resistance or Impedance of a mains supply.

In one aspect the present invention provides a tester and method for determining the line-earth loop resistance or Impedance of a mains supply which includes a line resistance, a neutral resistance and an earth resistance and can include a residual current detector (RCD), such as an earth-leakage circuit breaker, the method comprising the steps of: measuring line-neutral voltages both with and without a high-current load being applied across the : *.., line and neutral resistances of the mains supply, and determining the line- neutral resistance from the measured line-neutral voltages; measuring neutral-earth voltages both with and without a low current being injected into the neutral and earth resistances of the mains supply, and determining a difference resistance, which represents a difference between the earth : resistance and the neutral resistance, from the measured neutral-earth * voltages; and determining the line-earth resistance from the sum of the **.* line-neutral resistance and the difference resistance.

In another aspect the present invention provides a tester and method for determining the line-earth loop resistance or impedance of a mains supply and at least one other characteristic of the mains supply which can incorporate a residual current detector (RCD), such as an earth-leakage circuit breaker, the method comprising the steps of: determining a first characteristic of the mains supply; presenting the first characteristic of the mains supply to a user; determining a line-earth loop resistance as a second characteristic of the mains supply while, preferably simultaneously, one or both of determining and presenting the first characteristic of the mains * supply; and subsequently presenting the determined line-earth loop resistance of the mains supply to the user.

With this configuration, the user is rapidly, and preferably immediately, presented with the line-earth loop resistance on request. As the lineearth loop resistance Is determined while one or both of determining and presenting the first characteristic of the mains supply, there is no appreciable delay In the presentation of this resistance.

A preferred embodiment of the present invention will now be described hereinbelow by way of example only with reference to the accompanying drawings, in which: Figure 1 schematically Illustrates the application of a line-earth loop resistance tester in accordance with a preferred embodiment of the present : *", invention to an exemplary TN mains supply; ***. e

Figure 2 illustrates the main control circuitry of the tester of Figure 1; I S. * . . * S. Figure 3(a) illustrates the loading circuitry of the tester of Figure 1; Figure 3(b) illustrates the battery supply circuitry of the tester of Figure 1; S. II

S S S...

Figures 4(a) to (f) illustrate the measurement circuitry of the tester of Figure 1; and Figure 5 Illustrates the analogue-to-digital circuitry of the tester of Figure 1.

Figure 1 illustrates an exemplary mains supply 1, in this embodiment a TN mains supply, to which the line-earth loop resistance tester 3 of the present invention is connected.

The mains supply 1 comprIses a sub-station transformer 5, which supplies a mains voltage at the secondary thereof, a line resistance RL, which is the effective spur resistance at the line wire, a neutral resistance RN, which is the effective spur resistance at the neutral wire, and an earth resistance RE, which is the effective spur resistance at the earth wire, and a residual current detector (RCD) 7, which is set to trip on detection of a predetermined residual current, tyically between 15 mA and 30 mA.

The tester 3 comprises main control cIrcuitry 11, as illustrated in Figure 2, for controlling the. operation of the tester 3, loading circuitry 15, as illustrated in Figure 3(a), for connection to the line, neutral and earth resistances RI, RN, RE of the mains supply 1, battery supply circuitry 17, as illustrated in Figure 3(b), for connection to a battery supply (not illustrated), measurement circuitry 19, as illustrated In FIgure 4, for measuring the line, neutral and earth voltages VI, VN and VE at the line, neutral and earth resistances RL, RN, RE, and analogue-to-digital circuitry 25, as illustrated in : ** Figure 5, for providing a high-precision digital output for the earth voltage VE, as will be described in more detail hereinbelow. * *

In this embodiment the tester 3 is configured to accommodate an input voltage range for the line voltage VL of at least 360 V, in order to accommodate the full range of the line-neutral voltage VLN as seen In most mains supplies, and particularly throughout the EU. S... * S S...

In this embodiment the tester 3 is configured to accommodate an input voltage range for the earth voltage VE of about 8.5 V1 in order to accommodate the combination of a standing voltage which is principally due to current at the neutral resistance RN, and a measurement voltage which is principally developed across the earth resistance RE, and is adequate for domestic and light industrial applications.

In this embodiment the main control circuitry 11 utfltzes a mixed signal microcontroller 31, here a microprocessor, in this embodiment Including an integrated analogue-to-digital converter, here a 12 bit analogue-todigital converter, which operates under software control, and Includes a display 35 for displaying inter a/Ia determined line-earth resistances RLE.

In this embodiment the line and earth voltages VL, VE are measured using respectIve ones of the op-amps U1:A and U1:B of the measurement circuitry 19 (Figures 4(a) and (b)) to provide line and earth voltage signals VLX, VEX.

In this embodiment the line voltage signal VLX Is sampled and converted by the integrated analogue-to-digital converter of the microcontroller 31.

In this embodiment the earth voltage signal VEX Is sampled and converted by the analogue-to-digital converter 0101 of the analogue-to-digital circuitry in order to provide a high-precision, digital output SDI16 for the microcontroller 31.

: In this embodiment, using the neutral voltage VN as a OV voltage reference, positive and negative sampling signals IN_POS, IN_NEG are provided for the earth voltage signal VEX (Figure 4(c)), which sampling signals IN_POS, IN_NEG are sampled and converted by the analogue-todigital circuitry 25, as will be described in more detail hereinbelow.

In this embodiment the comparator U4 of the measurement circuitry 19 S...

(Figure 4(d)) generates zero crossing edge signals ZC which are utilized to synchronize the microcontroller 31, as will be described in more detail hereinbelow.

Operation of the line-earth resistance tester 3 will now be described herein below.

In a first stage, the mains supply 1 is subjected to a high-current test to determine the line-neutral resistance RLN (RL + RN).

In this embodiment, as will be described in more detail hereinbelow, at least two measurements of the line-neutral voltage VLN are made in order to allow for averaging.

In each measurement, the line-neutral voltage yIN is measured both without and with a high-current load being applied across the line and neutral resistances RI, RN of the mains supply 1. These measured line- neutral voltages VLN enable a determination of the voltage drop in the lineneutral voltage yIN, which, when divided by the test current, yields the line- neutral resistance RIN (RL + RN).

The high-current loading of the mains supply 1 is effected by the loading circuitry 15, in this embodiment via the fuse FU1, the loading resistor R14, the rectifier Dl and the thyristor SCR1, and the vanstor VDR2 limits transients, here to 800 V in the worst case, thus complying with regulatory requirements, for example, EN61O1O-1 300 V Cat LU in the EU. * 4* * * .

*.* The fuse FU1 is designed to last the service lifetime of the tester 3, while protecting principally against shorting of the thyristor SCR1, which might otherwise cause a fire hazard at the loading resistor R14. * **.

The loading resistor R14 is capable of about fifty successive tests before cooling is necessary, with the temperature of the loading resistor R14 being monitored by the thermistor R15.

The thyristor SCR1 is gate driven via the amplifier Qi, which is triggered by the microcontroller 31 in response to the zero crossing edge signals ZC of the mains supply 1, as will be described in more detail hereinbelow.

In this embodiment the line voltage signal VLX is sampled by the integrated analogue-to-digital converter of the microcontroller 31, with the sampling being controlled by the microcontroller 31 and synchronised to the zero crossing edge signals ZC of the mains supply 1.

In this embodiment the sampling frequency Is approximately 1200 Hz, and is frequency locked to reduce beat effects.

In this embodiment sampling occurs In a high priority regular interrupt and raw data is buffered in a queue to limit hard, real-time processing requirements. In a lower-priority process, the controlling software can apply calibration factors to reconstitute the voltage waveforms for subsequent processing as required.

In this embodiment two sets of data are captured for each measurement, each during a positive half-cycle.

In this embodiment the thyristor SCR1 is switched to an OFF state during : *, the first positive half-cycle, such that the mains supply 1 is unloaded, and *...

an ON state during the second positive half cycle, such that the mains supply 1 is loaded. * ** * * * * **

* In this way, measurements are obtained for unloaded and loaded halfcycles, which measurements are utilized to determine the line-neutral voltage VLN both without and with a high-current load being applied across the line and neutral resistances RL, RN of the mains supply 1. As set out hereinabove, these determined line-neutral voltages VLN enable a determination of the voltage drop in the line-neutral voltage VLN, which, when divided by the test current, yields the line-neutral resistance RLN (RL + RN).

Following these first two measurements, the measurements are subjected to a quality assessment against predetermined criteria. Where the quality assessment is satisfied, then no further measurements are conducted, but, where the quality assessment is not satisfied, a predetermined number of further measurements are conducted, in this embodiment up to a maximum of five measurements, in order to allow for further averaging.

In this embodiment the mean and standard deviation for the measurements are calculated, and, from the calculated mean, a working error budget is determined which, in this embodiment, is slightly less than half that of a predetermined budget for the tester 3. By setting the working error budget to be less than half of the predetermined budget, the tester 3 can accommodate two standard deviations of error in addition to the error build- up which occurs when the line-earth resistance RLE, as determined from the subsequent low-current test, Is incorporated, as will be described in more detail hereinbelow.

Where the standard deviation Is less than the working error budget, no more measurements of the line-neutral voltage VLN are conducted and the : ** averaged mean for the existing measurements is utilized. **.. * **.

Where the standard deviation is greater than the working error budget, a predetermined number of further measurements of the line-neutral voltage VLN are conducted, in this embodiment up to a maximum of five measurements. In this embodiment the measurements which represent the upper and lower bounding results are discarded, and the remaining **** measurements are processed in the same manner as set out hereinabove to provide an assessment of the quality of the measurements. Where the standard deviatIon determines that the mean is still noisy, then, when the determined value of the line-earth resistance RLE is ultimately displayed on the display 35, a flag is included to represent an out-of- bound level of noise.

In a second stage, the mains supply 1 is subjected to a low-current test to determine a difference resistance RE - RN, which represents a difference between the earth resistance RE and the neutral resistance RN.

In this embodiment, as wifl be described in more detail hereinbelow, at least two measurements of the neutral-earth voltage VNE are made in order to allow for averaging.

In each measurement, the neutral-earth voltage VNE is measured both without and with a low current being injected into the neutral and earth resistances RN, RE of the mains supply 1. These measured neutral-earth voltages VNE enable a determination of the voltage drop In the neutralearth voltage VNE, which, when divided by the test current, yields the difference resistance RE - RN.

A low current is injected into both the neutral and earth resistances RN, RE by the loading circuitry 15, in this embodiment through respective ones of the loading resistors R18, R19 via a single opto-etectronic switch U5, and the varistor VDR1 protects the opto-electronic switch US against transients, : *. in this embodiment as further assisted by the loading resistor R14 and the varistor VDR2. S...

*:*::* The loading resistors R18, R19 are rated for 100 % duty in case of short circuiting of the opto-etectronic switch U5, with the temperatures of the *:*. loading resistors R18, R19 being monitored by the thermistor R20. S.. .

In this embodiment the earth voltage signal VEX, where provided as sampling signals IN..30S, IN_NEG, Is sampled and converted by the analogue-to-digital converter UlOl of the analogue-to-digital circuitry 25, which Is external to the microcontroller 31, with the sampling being controlled by the microcontroller 31 and synchronised to the zero crossing edge signals ZC of the mains supply 1.

In this embodiment sampling occurs in a high priority regular interrupt and raw data is buffered In a queue to limit hard, real-time processing requirements. In a lower-priority process, the control software can apply calibration factors to reconstitute the voltage waveforms for subsequent processing as required.

In this embodiment four. sets of data are captured for each measurement, each set consisting of a consecutive half-cycle of the mains supply 1. In this embodiment fifty pairs of cycles are sampled, which at 50 Hz requires a sampling time of 2 5. Owing to the differential effect of comparing adjacent cycles, the noise is particularly suited to averaging, as white noise becomes violet and the square root rule is pessimistic. In this embodiment the microcontroller 31 allows for the processing of all of the sampled mains cycles.

In this embodiment the opto-electronic switch 05 is switched to an OFF : ** state during the first cycle of each cycle pair, such that no current is injected S...

into the neutral and earth resistances RN, RE, and switched to an ON state * I S...

during the second cycle of each cycle pair, such that a low current is injected into the neutral and earth resistances RN, RE.

S I..

In this way, measurements are obtained for cycles both without and with *:*. low-current Injection, which measurements are utilized to determine the I. .5 neutral-earth voltage VNE both without and with a low current being injected Into the neutral and earth resistances RN, RE of the mains supply 1.

As set out hereinabove, these determined neutral-earth voltages VNE enable - 10 - a determination of the voltage drop in the neutral-earth voltage VNE, which, when divided by the test current, yields the difference resistance RE - RN.

In this embodiment, because of the possibility of direct current (DC) in the neutral-earth voltage VNE and the requirement for a linear method to eliminate standing voltages, the neutral-earth resistances RNE are first separately determined for each of the positive and negative half cycle pairs, and then subsequently combined.

In this embodiment the data will have a low signal-to-noise ratio, and the data is pre-fittered by the microcontroller 31 before performing a quality assessment. In a preferred embodiment the pre-filtering time constants are lengthened signifIcantly for the second and any subsequent measurements.

By lengthening the pre-fuitering time constants, a better quality result is achieved with the additional data from the second and subsequent measurements.

Following the two measurements, the measurements are subjected to a quality assessment against predetermined criteria in the same manner as described hereinabove in relation to the high-current test. Where the quality assessment is satisfied, then no further measurements are conducted, but, where the quality assessment is not satisfied, a predetermined number of further measurements are conducted, in this embodiment up to a maximum of five measurements, in order to allow for further averaging. * * * **

Following the determination of the line-neutral resistance RLN (RL + RN) * and the difference resistance RE - RN, which represents a difference between the earth resistance RE and the neutral resistance RN, the linep. earth resistance RLE (RL + RE) is determined from the sum of the lineneutral resistance RLN (RL + RN) and the difference resistance RE - RN.

- 11 - The determined line-earth resistance RLE is then displayed on the display 35, and, as described hereinabove, where the associated data measurements are noisy, as represented by the standard deviation exceeding a predetermined working budget, a flag is included in the display 35.

In this embodiment the tester 3 is configured to provide information as regards at least one other characteristic of the mains supply 1, such as a mains wiring analysis, prior to the display of the determined line- earth resistance RLE for the mains supply 1. This configuration of the tester 3 is particularly advantageous in that the determination of the line-earth resistance RLE can be performed simultaneously with the determination of the at least one other characteristic, thus allowing for the rapid, and In most circumstances immediate, display of the determined line-earth resistance RLE. In addition, this configuration allows for the capturing of greater numbers of data measurements, as may be necessary to achieve required specifications when operating in noisy environments.

Finally, it will be understood that the present invention has been described In its preferred embodiment and can be modified in many different ways without departing from the scope of the invention as defined by the appended claims. * I IS.. S..

I

ISS I I. * .

S S.. * I S. S.

S S..

Claims

- 12 - CLAIMS

1. A tester for determining the line-earth loop resistance or impedance of a mains supply which includes a line resistance, a neutral resistance and an earth resistance, the tester comprising: measurement means for measuring line-neutral voltages both with and without a high-current load being applied across the line and neutral resistances of the mains supply, and neutral-earth voltages both with and without a low current being injected into the neutral and earth resistances of the mains supply; and determination means for determining the line-neutral resistance from the measured line-neutral voltages, a difference resistance, which represents a difference between the earth resistance and the neutral resistance, from the measured neutral-earth voltages, and the line- earth resistance from the sum of the line-neutral resistance and the difference resistance.

2. The tester of claim 1, wherein the mains supply includes a residual current detector (RCD), such as an earth-leakage circuit breaker.

3. The tester of claim 1 or 2, wherein the measurement means is configured to measure the line-neutral voltages with and without a load being applied across the line and neutral resistances of the mains * supply during the positive half-cycles of respective cycles. **s

*

4. The tester of claim 3, wherein the cycles are consecutive cycles. ** S * I * * **

5. The tester of claim 3 or 4, wherein the measurement means is **** configured such as to provide that the mains supply is unloaded during a first positive half-cycle and measure the line-neutral voltage when the mains supply Is so unloaded, and load the mains supply during a second positive half-cycle and measure the line-neutral voltage when the mains supply is so loaded.

- 13 -

6. The tester of any of claims 1 to 5, wherein the measurement means is configured to capture at least two sets of measurements for the line- neutral voltages.

7. The tester of claim 6, wherein the determination means is configured to determine a mean value for a predeterminable number of sets of first measurements for the line-neutral voltages, and, where the mean value for the sets of first measurements does not satisfy predeterminabie criteria, the measurement means is configured to capture a predeterminable number of further sets of measurements for the line-neutral voltages.

8. The tester of any of claims 1 to 7, wherein the measurement means Includes a loading resistor and a thyristor which is triggered such as selectively to connect the loading resistor across the line and neutral resistances.

9. The tester of claim 8, wherein the thyristor Is triggered in response to zero crossing edge signals of the mains supply.

10. The tester of any of claims 1 to 9, wherein the measurement means is configured such that the neutral-earth voltages with and without a * ** low current being Injected Into the neutral and earth resistances of * * * * *.

* the mains supply are measured in the respective cycles of pairs of cycles. ** * * * * * **

11. The tester of claim 10, wherein each pair of cycles comprise **** consecutive cycles.

12. The tester of claim 10 or 11, wherein the measurement means Is configured such as to provide that no current is injected into the neutral and earth resistances in a first cycle of a cycle pair and - 14 - measure the neutral-earth voltage when no current is so Injected, and inject current into the neutral and earth resistances in a second cycle of the cycle pair and measure the neutral-earth voltage when current is so injected.

13. The tester of any of claims 10 to 12, wherein four sets of data are measured for each pair of cycles, comprising data from each half- cycle.

14. The tester of claim 13, wherein the determination means is configured separately to determine the neutral-earth resistances for each of the positive and negative half-cycle pairs, and subsequently combine the neutral-earth resistances as so determined.

15. The tester of any of claims 10 to 14, wherein the measurement means Is configured such that the neutral-earth voltages are measured in at least fifty cycle pairs.

16. The tester of any of claIms 1 to 15, whereIn the measurement means is configured to capture at least two sets of measurements for the : neutral-earth voltages. **..

17. The tester of claim 16, whereIn the determination means is configured * ** to determine a mean value for a predeterminable number of sets of first measurements for the neutral-earth voltages, and, where the mean value for the sets of first measurements does not satisfy predeterminable criteria, the measurement means is configured to capture a predeterminable number of further sets of measurements I...

for the neutral-earth voltages.

18. The tester of any of claims 1 to 17, wherein the measurement means comprises first and second loading resistors through which a low current is injectable Into both the neutral and earth resistances, and - 15 - an opto-electronic switch which is operable selectively to provide for the injection of current to the neutral and earth resistances through the first and second loading resistors.

19. The tester of claim 18, wherein the opto-electronic switch is triggered in response to zero crossing edge signals of the mains supply.

20. The tester of any of claims 1 to 19, further comprising: display means for displaying the determined line-neutral resistance.

21. A method of determining the line-earth loop resistance or impedance of a mains supply which includes a line resistance, a neutral resistance and an earth resistance, the method comprising the steps of: measuring line-neutral voltages both with and without a high-current load being applied across the line and neutral resistances of the mains supply; measuring neutral-earth voltages both with and without a low current being Injected into the neutral and earth resistances of the mains supply; determining the line-neutral resistance from the measured line- :.:::, neutral voltages; **S* determining a difference resistance, which represents a difference * ** between the earth resistance and the neutral resistance, from the measured neutral-earth voltages; and *** determining the line-earth resistance from the sum of the line-neutral *. : resIstance and the difference resistance. * S. *.*. * . S...

22. The method of claim 21, wherein the mains supply includes a residual current detector (RCD), such as an earth-leakage circuit breaker.

23. The method of claim 21 or 22, wherein the step of measuring the lineneutral voltages comprises the step of: - 16 - measuring the line-neutral voltages with and without a load being applied across the line and neutral resistances of the mains supply during the positive half-cycles of respective cycles.

24. The method of claim 23, wherein the cycles are consecutive cycles.

25. The method of claim 23 or 24, wherein the mains supply Is unloaded during a first positive half-cycle and loaded during a second positive half-cycle.

26. The method of any of claims 21 to 25, wherein at least two sets of measurements are captured for the line-neutral voltages.

27. The method of claim 26, further comprising the step of: determining a mean value for a predeterminable number of sets of first measurements for the line-neutral voltages; and where the mean value for the sets of first measurements does not satisfy predeterminable criteria, capturing a predeterminable number of further sets of measurements for the lineneutral voltages.

28. The method of any of claims 21 to 27, wherein the step of measuring the line-neutral voltages comprises the step of: triggering a thyristor such as selectively to connect a loading resistor * .* across the line and neutral resistances. * . . * S. * S..

29. The method of claim 28, whereIn the thyristor is triggered In response *. to zero crossing edge signals of the mains supply. *SSS * S *5*S

30. The method of any of claims 21 to 29, wherein the step of measuring the neutral-earth voltages comprises the step of: measuring the neutralearth voltages with and without a low current being injected into the neutral and earth resistances of the mains supply in the respective cycles of pairs of cycles.

- 17 -

31. The method of claim 30, wherein each pair of cycles comprise consecutive cycles.

32. The method of claim 30 or 31, wherein no current is injected into the neutral and earth resistances In a first cycle of a cycle pair and current is injected into the neutral and earth resistances in a second cycle of the cycle pair.

33. The method of any of claims 30 to 32, wherein four sets of data are measured for each pair of cycles, comprising data from each half- cycle.

34. The method of claim 33, whereIn the step of determining the neutralearth resistance comprises the step of: separately determining the neutral-earth resistances for each of the positive and negative halfcycle pairs, and subsequently combining the neutral-earth resistances as so determined.

35. The method of any of claims 30 to 34, wherein the neutral-earth : voltages are measured in at least fifty cycle pairs.

36. The method of any of claims 21 to 35, wherein at least two sets of * measurements are captured for the neutral-earth voltages.

37. The method of claim 36, further comprising the step of: determining a mean value for a predeterminable number of sets of first measurements for the neutral-earth voltages; and *** where the mean value for the sets of first measurements does not satisfy predeterminable criteria, capturing a predeterminable number of further sets of measurements for the neutralearth voltages.

- 18 -

38. The method of any of claims 21 to 37, whereIn the step of measuring the neutral-earth voltages comprises the step of: selectively triggering an opto-electronic switch to provide for the injection of current to the neutral and earth resistances through first and second loading resistors.

39. The method of claim 38, whereIn the opto-electronic switch is triggered In response to zero crossing edge signals of the mains supply.

40. The method of any of claims 21 to 39, further comprising the step of: displaying the determined line-neutral resistance.

41. A tester for determining the line-earth loop resistance or impedance of a mains supply and at least one other characteristic of the mains supply, comprising: display means for displaying a first characteristic of the mains supply and subsequently a line-earth loop resistance as a second characteristic of the mains supply; and determining means for determining the first characteristic of the * mains supply, and, while displaying the first characteristic of the mains supply, at least partially determining the line-earth loop **** resistance as the second characteristic of the mains supply. * S.

42. The tester of daim 41, whereIn the mains supply includes a residual S. .

current detector (RCD), such as an earth-leakage circuit breaker. *5 * * . . * *.

43. The tester of claim 41 or 42, wherein the first characteristic of the S...

mains supply Is a wiring analysis.

44. The tester of any of claims 41 to 43, wherein the line-earth loop resistance as the second characteristic of the mains supply is determined subsequently to the first characteristic.

- 1.9 -

45. The tester of any of claims 41 to 44, wherein the line-earth loop resistance as the second characteristic of the mains supply is determined while the first characteristic of the mains supply is displayed.

46. A method of determining the line-earth loop resistance or Impedance of a mains supply and at feast one other characteristic of the mains supply, the method comprising the steps of: determining a first characteristic of the mains supply display; displaying the first characteristic of the mains supply; determining the line-earth loop resistance of the mains supply as a second characteristic of the mains supply while at feast partially displaying the first characteristic of the main supply; and subsequently displaying the tine-earth loop resistance as the second characteristic of the mains supply.

47. The method of claim 46, wherein the mains supply includes a residual current detector (RCD), such as an earth-leakage circuit breaker.

48. The method of claim 46 or 47, whereIn the first characteristic of the * *.

mains supply is a wiring analysis. * **e * S * .SS

*:*::*

49. The method of any of claims 46 to 48, wherein the line-earth loop * resistance as the second characteristic of the mains supply Is SI.

determined subsequently to the first characteristic. I. S * S.

50. The method of any of claims 46 to 49, wherein the line-earth loop resistance as the second characteristic of the mains supply is determined while the first characteristic of the mains supply is displayed.

- 20 -

51. A tester for determining the line-earth loop resistance or impedance of a mains supply substantially as hereinbefore described with reference to the accompanying drawings.

52. A method of determining the line-earth loop resistance or impedance of a mains supply substantially as hereinbefore described with reference to the accompanying drawings.

53. A tester for determining the line-earth loop resistance or impedance of a mains supply and at least one other characteristic of the mains supply substantially as hereinbefore described with reference to the accompanying drawings.

54. A method of determining the line-earth loop resistance or impedance of a mains supply and at least one other characteristic of the mains supply substantially as hereinbefore described with reference to the accompanying drawings. * *. * * S S...

55.5 * I * 5** * 5S

SI I * I.

S S..

S SI * * S. * SS * S *S..